Does the United States now follow a moral idealist or a political realist approach in international affairs?
First, it is important to realize that it is very difficult to determine whether a country is acting out of idealism or realism. To some degree, the distinction between these two is found in motivations and intentions, not in actions. In addition, political leaders sometimes give contradictory arguments for their actions. For example, when President Bush pushed for an invasion of Iraq, he talked in both idealist and realist terms. He talked about spreading democracy, which is an idealist view of how to make the world more peaceful. However, he also talked about the danger of weapons of mass destruction, which is a realist, security-based argument. Which of these was the “real” reason we went to war against Iraq? It is hard to know.
That said, I would argue that the US today is more inclined to realism than to idealism. We can, arguably, see this in our attitude towards Syria. There, we are standing by while a horrific civil war rages. Idealists might argue that we should get involved and back the most moderate of the people who are fighting against Assad. This would make it more likely that a new Syria would have a liberal democracy. Realists would likely argue that Syria poses no threat to us regardless of who rules it. We are therefore better-off not getting involved. We can also argue that this is a major reason why we are leaving Afghanistan. We went there in part to build a viable nation and we have not done so. This means we have failed in our idealist mission. But we are leaving now, in part because we feel that we are spending too many lives and too much money in a country that is only vaguely connected to our national interests. Finally, our widespread use of drones to kill our enemies even in friendly countries can be seen as more evidence of our embrace of realism.
Thus, though we cannot be absolutely certain, I would argue we are more realist than we are idealist today.